This invention relates to battery operated circuits and, more particularly, to a method and system for detecting low battery voltage utilizing a stable reference voltage and a switched-capacitor circuit.
In many battery operated applications, some means for detecting a low battery voltage is needed. In such applications, a warning signal might be generated or, alternatively, vulnerable circuits within a system might be selectively disabled by an appropriate control signal in order to avoid damage to or malfunction of such circuits. A straightforward method of implementing a low voltage detection function is to compare a scaled battery voltage and a stable reference voltage such as a voltage from a bandgap circuit. It is known, as will be discussed in greater detail later, that a bandgap circuit can generate a stable, substantially temperature independent reference voltage. Normal time-continuous bandgap references typically need well-matched resistors and transistors and some sort of trimming to get good accuracy.
U.S. Pat. No. 5,196,833, which issued Mar. 23, 1993, to Kemp describes a low voltage detection circuit which includes a bandgap reference and a differential comparator circuit for generating a signal proportional to the level of the supply voltage. Since many battery operated circuits are subject to ambient temperature variations, such as an automobile application as disclosed in the U.S. Pat. No. 5,196,833, it is important that some form of temperature compensation be used in deriving the reference voltage. The bandgap cell, as disclosed in the aforementioned patent, includes bipolar transistors in which the base to emitter voltage is used as a reference source. Typically, two transistors are used having different current densities through each. In the U.S. Pat. No. 5,196,833, the different current densities are achieved by using four parallel transistors in one current path and a single transistor in the second. It has been shown that the base to emitter voltage (VBE) of a bipolar transistor exhibits a negative temperature coefficient with respect to temperature. On the other hand, it has also been shown that the difference of base to emitter voltages xcex94VBE of the two bipolar transistors operating at different current densities exhibit a positive temperature coefficient with respect to temperature. Thus, the sum of the base to emitter voltage VBE of a bipolar transistor and a differential voltage xcex94VBE will be relatively independent of temperature when the sum of the voltages equals the energy gap of silicon. Such temperature stable references have been created by generating a VBE and summing a xcex94VBE of such value that the sum substantially equals the bandgap voltage of 1.205V.
U.S. Pat. No. 5,814,995, which issued Sep. 29, 1998, to Tasdighi, discloses a voltage detector for a battery operated device employing two bipolar transistors and an operational amplifier as a comparator. The system of this reference also employs resistors which, as noted hereinbefore, must be well-matched and usually require some sort of trimming to obtain good accuracy.
U.S. Pat. No. 4,375,595, which issued Mar. 1, 1983, to Ulmer et al., relates to a temperature independent bandgap reference which employs switched capacitors to input the VBE and xcex94VBE of the bipolar transistors. A proper selection of the ratio of the switched capacitors is used to get around the need for matched and/or trimmed resistors. The switched capacitor implementation employs clock signals in order to establish a precharge phase and an output reference stage. In the U.S. Pat. No. 4,375,959, three separate clock signals are required.
U.S. Pat. No. 5,563,504, which issued Oct. 8, 1996, to Gilbert et al., also relates to a switching bandgap voltage reference in which one bipolar transistor is used with two different current sources providing the current densities needed to obtain the xcex94VBE value.
Thus, in an application where a battery voltage detector is required in order to detect a low voltage value, and providing that a suitable clock exists within the application, a switched capacitor architecture can provide an effective solution.
The present invention provides lower untrimmed errors by utilizing offset cancellation techniques. This, in combination with a switched capacitor network having well-matched capacitors provide suitable weighting factors.
The invention also provides for a reduced component count by combining a voltage reference circuit and a voltage comparator into a single circuit.
In a preferred embodiment, the comparator implementation eliminates the need to actually generate a 1.2V bandgap reference voltage.
The invention also provides for lower power requirements by eliminating part of the static current by replacing resistors in the bandgap circuit with a switched capacitor circuit.
Therefore, in accordance with a first aspect of the present invention there is provided a detector for providing a low battery voltage indication comprising: a stable voltage reference and comparator circuit for comparing the battery voltage and the voltage reference; switch means to switch between a first operational state and a second operational state; a switched capacitor circuit to store charges related to the battery voltage and the reference voltage in each operational state; and a clock to initiate switching between the first and second states, wherein an output from the comparator during the second state indicates whether the battery voltage is below a preset threshold.
In accordance with a second aspect of the present invention there is provided a method of detecting a low battery voltage supplied to an integrated circuit comprising: providing a stable voltage reference and comparator circuit for comparing battery voltage against the reference voltage; providing a capacitor circuit for storing charges associated with the battery voltage and the reference voltage; providing switching means to switch between a first phase wherein the capacitors are charged to a first voltage level, and a second phase wherein the capacitors are charged to a second level; and comparing the first and second levels to determine whether the battery voltage is above or below a preset threshold.